Vegetation Responses and Restoration

Completed in 2009Vegetation assessment Surveys were conducted throughout the lower Virgin Valley to establish pre-beetle baseline of plant species composition and cover, with particular emphasis on vegetation architecture as it concerns wildlife habitat. Then, in the fall of 2009, 24 cross-sectional profiles were established between Littlefield, AZ and the mouth of the Virgin River at Lake Mead to assess vegetation composition and structure and underlying channel topography including erosion surfaces, for comparison following Diorhabda establishment. Biocontrol impact Individual permanently marked tamarisk trees were monitored bi-weekly to document seasonal patterns of plant growth, at most sites to provide pre-beetle baseline data on vegetation dynamics but at some sites whereDiorhabda has colonized, to document initial response to biocontrol herbivory. Data include physiological condition, regrowth and dieback status, and associated plants. Restoration A series of vegetation planting treatments were installed Spring 2009 at mesic (cottonwood-willow), and drier (mesquite-Acacia) sites to test approaches to rapid and effective restoration where defoliation has potential risks. In Fall 2009 extensive plantings were conducted at two locations using similar treatments, with mesquite and catclaw acacia at one site downstream of Riverside, and zones with both xeric taxa and adjacent in wetter soils, cottonwoods and willows, adjacent to Hughes Middle School in Mesquite. We also surveyed streamside vegetation from Littlefield, AZ to the Riverside Bridge to determine where Fremont cottonwood recruitment occurs in relation to mature, reproductive cottonwood stands. These data will be used to determine spatial distribution of the “restoration islands” model described below.

Completed in 2010Vegetation assessment Cross-sectional vegetation transects were re-surveyed during the spring and fall in 2010 (and subsequent years) to assess changes in diversity, abundance and architectural habitat of riparian vegetation. Sediment transport models were applied to determine effect of vegetation change on channel stability, or how sediments were deposited or eroded and exported downstream. This data was included in remotely sensed databases from the SNWA to provide geo-spatial characterization of floodplain features, including landscape-level changes in tamarisk total cover, water transpiration information, etc. Responses of tamarisk and native species to wildfire or prescribed fire continued, including assessment of the role of tamarisk in fueling fires and damaging native habitat across the Mojave region. Biocontrol impact Marked tamarisk trees were regularly monitored to track Diorhabda colonization and impacts to target trees. Tissue samples were collected to assay soluble carbohydrates as an index of herbivore-induced stress to predict mortality. The zone of defoliation was mapped and quantified from ground and remote sensing (e.g. NDVI) methods. Data yielded comprehensive assessment of habitat value and fire risk during the course of the 2010 season. Post-biocontrol soil analyses documented changes in nutrient or chemistry (esp. salinity) as a consequence of defoliation.Restoration Restoration trials were continued in 2010, and by developing a spatial framework for establishing “restoration islands”. These are re-vegetated patches protected from grazing, to serve as seed sources for subsequent natural germination following periodic flood events. The data collected in 2009 on cottonwood recruitment helped identify key locations to implement islands so as to maximize the probability of providing seed sources to areas when the right post-flood conditions for germination arise.

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